Leaching solution and metal recovery method

ABSTRACT

A valuable metal recovery method of recovering metals from a lithium ion battery without using complicate steps and by a relatively simple and convenient facility is intended to be provided. 
     For attaining the purpose, lithium is leached selectively from a positive electrode active material containing a composite oxide of lithium and transition metal elements by using a solution showing a weak acidity at a pH of 4 to 7 so that the high Li/Co selectivity is high and a Li recovery rate is high, and lithium is recovered from the leaching solution. By using a solute that the acidity of the acidic solution spontaneously disappears due to evolution of a gas after leaching of lithium, neutralization step is no more required and the volume of liquid wastes is decreased.

TECHNICAL FIELD

The present invention concerns a metal recovery technique of recovering metals from lithium ion batteries simply and conveniently.

BACKGROUND

Along with recent progress in portability of electronic equipment, use of secondary batteries has been increased rapidly. As application of the secondary batteries extends not only to equipment of relatively low power consumption such as mobile phones or portable music players but also to equipment requiring high power such as electric tools, electric bicycles, and electric automobiles, lithium ion batteries capable of providing high energy density have attracted attention. Along with increasing application to high power equipment, necessity for recovering of valuable materials from waste batteries has been increasing and various techniques have been proposed for the recovery of valuable metals from the lithium ion batteries.

A recycling technique for lithium ion batteries is featured and a method of recovering valuable metals constituting lithium ion batteries is described systematically, for example, by Jinqiu Xu et al., “A review of processes and technologies for the recycling of lithium-ion secondary batteries”, Journal of Power Sources, vol. 177, pp. 512-527 (2008) (non-patent document 1). According to a typical recycling method described in the non-patent document 1, waste lithium ion batteries are subjected to mechanical treatments such as unsealing, disassembling, and pulverization, then a positive electrode active material containing valuable metals is entirely dissolved by acid leaching, from which desired ingredients are separated and recovered for every ingredient by a treatment, for example, of separating them for every ingredient and precipitating by utilizing the difference of dissolving properties for every ingredient, or subjecting a desired ingredient preferentially to solvent extraction.

Further, Japanese Patent No. 3675392 discloses a technique of recovering copper and cobalt by diaphragm electrolysis using a solution formed by dissolving valuable metals obtained by acid leaching as a cathode solution and using a cation exchange film as a diaphragm. In the present specification, a liquid before treating the valuable metal is referred to as a leaching solution and a liquid after treating the valuable metal is referred to as a dissolved solution.

SUMMARY

In the non-patent document 1 by Jinqiu Xu et al., it is intended to attain a compatibility between improvement in the recovery rate of the valuable materials and high purity of recovered products, but there is a large room for the improvement since treatment steps are complicate, as well as an enormous investment cost is necessary for treating a great amount of waste batteries.

Further, Japanese Patent No. 3675392 specifically uses a facility of utilizing ion selectivity of a cationic exchange film (diaphragm electrolysis cell illustrated in FIG. 2 of JP No. 3675392) and diffusion dialysis equipment utilizing the anion selectivity of an anion selection membrane (with no explanatory view). Referring more specifically, main valuable metals can be recovered by a series of treatments including electrodeposition recovery of Cu by diaphragm electrolysis→pH control→electrodeposition recovery of cobalt by diaphragm electrolysis→pH control→settling recovery of Fe(OH)₃ and Al(OH)₃→recovery of Li₂Co₃ by adding a carbonate salt. According to the technique, since copper (bivalent ions) and cobalt (trivalent ions) are recovered by electrochemical reduction, metals at high purity can be obtained. However, there is still a room for improvement in that enormous amount of electricity should be applied in a case of treating a great amount of waste batteries.

For example, for recovering of about 100 kg of cobalt, it is necessary to continuously supply a current of 1 ampere for about 100 hours. However, since an approximately equal electric amount is applied also in the preceding copper electroanalysis, unexpected labors are necessary for recovering entire metals only by the diaphragm electrolysis. Further, since the amount of liquid increases for every pH control in a multi-stage, a lithium concentration is lowered when Li₂CO₃ is recovered at the final stage in the series of treatments, and it is considered that the recovery rate of lithium is not always high even when a carbonate salt is added. This is because unrecovered ingredients increase more as the amount of liquid becomes larger since the saturated solubility of lithium carbonate is as high as 1.3 wt % at 20° C. In order to avoid such disadvantage, treatment, for example, addition of a concentration step is necessary. Further, since Fe(OH)₃ and Al(OH)₃ tend to be gelled in a weakly acidic to neutral aqueous solution, operation for the step of recovering Fe(OH)₃ or Al(OH)₃ by filtration based on the technique of Japanese Patent No. 3675392 is not easy. On the other hand, when the liquid is diluted for facilitating the filtration operation, the lithium recovery rate is lowered. Further, since the surface of gelled settling such as Fe(OH)₃ and Al(OH)₃ also has a property of adsorbing lithium ions, it is difficult to greatly improve the lithium recovery rate also from this view point.

A typical example of the inventions disclosed in the present application is to be described briefly as below.

Lithium is leached selectively from a positive electrode active material containing lithium and transition metal elements by using a weakly acidic leaching solution (pH at 4 to 7), to separate them into transition metal element ingredients as a not-leached solid content and a lithium ingredient in the leaching solution. Since an acidic solution in which a solute disappears spontaneously to lower the concentration is used, this treatment is a liquid waste-free process.

The present invention can provide a valuable metal recovery method of recovering valuable metals from lithium ion batteries at a high efficiency simply and conveniently.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates compositions of leaching solutions according to examples of the present invention and a Li/Co ratio as a result of analysis for the solution obtained by treatment with a leaching solution;

FIG. 2 illustrates a schematic step flow for recovery of valuable metals in the examples according to the invention;

FIG. 3 illustrates plots showing correlation between pH and redox potential due to concentration of an aqueous-hydrogen peroxide according to the example of the invention;

FIG. 4 illustrates compositions of leaching solutions according to the example of the present invention, and Li/Co ratio for the result of analysis on dissolved solution obtained by treatment with the leaching solution; and

FIG. 5 illustrates plots showing the relation between leaching time and dissolved oxygen concentration according to the example of the invention.

DETAILED DESCRIPTION

Preferred embodiments of practicing the present invention are to be described.

First Embodiment

The outline of a method of recovering valuable metals according to this embodiment is to be described with reference to FIG. 2. FIG. 2 is a schematic step flow for recovering valuable metals from a waste lithium battery (hereinafter referred to as waste battery) of this embodiment. First, respective constituent components obtained by disassembling a waste battery (S101) are sorted for every component (S102), and only the electrode active material containing valuable metals at a high concentration is taken out. The thus taken out electrode active material is treated by a solution for selectively leaching lithium (lithium selective leaching: S103) to form a solution in which lithium is leached. The solution containing selectively leached lithium and the not-leached content is put to solid-liquid separation (S104). When a carbonate salt or gaseous carbon dioxide is mixed with a solution A containing lithium (S105), lithium can be recovered as lithium carbonate Li₂CO₃ (S106). In the solid-liquid separation, a solid ingredient B is recovered (S107). When multiple transition metals are contained, they are recovered by filtration by precipitating and settling them sequentially as hydroxides by a simple and convenient operation of pH control after dissolving the solid ingredients B (S108). The valuable metals can be recovered from waste battery by the series of operations described above.

The valuable metal recovery flow is to be described more specifically in accordance with the steps illustrated in FIG. 2. For recovering valuable metals from a waste battery, it is at first necessary to disassemble the battery. The battery is discharged before disassembling since there may be a possibility that electric charges still remain in the battery. In this embodiment, charges remaining in the battery are discharged by immersing the battery in an electroconductive liquid containing an electrolyte.

Since lithium ions dispersed in the battery can be concentrated into the inside of the positive electrode active substance by the discharging operation, the amount of lithium recovery can be maximized. Further, lithium selectivity in the leaching treatment is at the maximum by ensuring a state in which lithium is taken into a specified crystal structure. When the positive electrode active substance is LiCoO₂, since it is said that when the positive electrode active material is LiCoO₂, it exists as Li_(0.4)CoO₂ in the completely charged state and as LiCoO₂ in the completely discharged state, there may be a worry of causing lithium recovery loss of about 60% at the maximum if the discharging treatment is not applied. Naturally, the discharging treatment also provides an advantage capable of ensuring safety in the battery disassembling step and the pulverization step.

In this embodiment, a mixed solution of sulfuric acid/γ-butyrolactone was used as a conductive liquid containing the electrolyte. Since the sulfuric acid acts as an electrolyte in the mixed solution, electroconductivity (reciprocal to resistance value) can be controlled by controlling the concentration of the sulfuric acid. In this embodiment, when the electric resistance of the solution was actually measured from the rightmost end to the leftmost end of a discharging vessel, it was 100 kΩ. If the resistance of the solution is excessively low, discharging progresses excessively rapidly to induce a danger and, on the other hand, if the resistance is excessively high, discharging takes much time to deteriorate practicability. In this embodiment, the solution resistance is preferably in a range of about 1 k to 1000 kΩ and the concentration of the electrolyte is preferably controlled so that the resistance value is within the range.

The waste battery used in this embodiment includes, in addition to so-called spent batteries which have reached the limit of predetermined charge/discharge cycles and the charge capacity has been lowered, semi-processed goods occurring due to failure in the battery manufacturing steps and old type inventory adjustment products occurring due to change of specification of products.

A waste battery after the discharge treatment is disassembled at S101. Battery constituent components of the waste battery after the discharge treatment such as casings, packings, safety valves, circuit devices, spacers, collectors, separators, and positive electrode and negative electrode active materials are disassembled and sorted for every component by using an appropriate method.

Since the waste lithium ion battery is often in a pressurized state with the gas being filled fully inside the battery, operational safety should be taken into consideration. In this embodiment, wet pulverization was performed while cooling them in a state dipped into the conductive liquid containing the electrolyte. The battery could be pulverized safely without scattering gases filled in the batteries into the atmospheric air by adopting the wet pulverization under cooling.

Further, for promoting peeling of the positive electrode active material and the negative electrode active material coated and formed on the surface of collectors from the surface of the respective collectors, the composition of the electrolyte-containing electroconductive liquid may be controlled. Electroconductivity is an important property to be noted in the electroconductive liquid used for the discharging step, and properties such as viscosity and dielectric constant should be noted in the electroconductive liquid used in the wet pulverization step. Since required specifications are different between the discharging step and the wet pulverization step, composition of the conductive liquid used may be changed for every step. In this case, two or more kinds of conductive liquids have to be prepared. In this embodiment, the electroconductive liquid had an identical composition with a view point of simplification and suppression of labors and cost.

The wet pulverization method usable in this embodiment includes, for example, a method of using a ball mill but this is not restrictive. A sieving treatment is applied after pulverizing the battery under the condition of preferentially pulverizing the electrode active material of the positive electrode (hereinafter referred to as positive electrode active material) and the electrode active material of negative electrode (hereinafter referred to as a negative electrode active material) among the constituent components such as casings, packings, safety valves, circuit devices, spacers, collectors, separators, and electrode active materials. Thus, the positive electrode active material and the negative electrode active material are sorted and recovered such that positive electrode active material and the negative electrode active material are undersize pulverizates and other materials are oversize pulverizates (S102).

While sieving was used in this embodiment, since the components are pulverized in the wet process, a slurry obtained by wet pulverization can be sorted as it is by a filtration treatment using a filter of a relatively large mesh. A recovery rate may be possibly improved by adopting a continuous treatment of wet pulverization-filtration. Casings, packings, safety valves, collectors (aluminum foil, copper foil), etc. have larger extendability and, accordingly, a larger rapture strength than the positive electrode active material (typically LiCoO₂) or negative electrode active material (typically graphite). Due to the property, pulverizates of the electrode active material have a size smaller than pulverizates obtained from other components and, as a result, can be easily sorted and recovered by sieving or filtration.

The under size pulverizates obtained by the treatment described above are subjected to a leaching treatment by a weakly acidic leaching solution (S103).

In this embodiment, a spent lithium ion battery for digital camera was disassembled. The positive electrode active material of the waste battery used in this embodiment was a lithium compound mainly comprising LiCoO₂ but it may also contain positive electrode active materials of other compositions such as iron phosphate, nickel, manganese, etc. The positive electrode active material was mixed with a weakly acidic leaching solution and stirred at a room temperature for one hour to leach lithium. In this embodiment, the reaction temperature and the reaction time in the lithium selective leaching step were controlled, and the leaching treatment was stopped specifically at a reaction ratio of 80% or less before complete dissolution of the positive electrode active material. With a practical point of view, the reaction rate is most preferably, about 70 to 75%. If it exceeds 80%, deterioration of the selectivity in the selective lithium leaching reaction may possibly increase and, on the other hand, if it is below 70%, the recovery rate is lowered to deteriorate economy.

In this embodiment, the leaching solution and the residue are separated for terminating the leaching treatment (S104). For the separation method, centrifugal separation, filtration, etc. can be adopted. In this embodiment, separation and recovery were performed by centrifugal separation at a room temperature, under 15,000 rpm, for 15 minutes. Separation between the leaching solution and the residue is facilitated at a higher number of rotations.

FIG. 1 illustrates the Li/Co molar ratio of the obtained dissolved solutions. As illustrated in FIG. 1, when the positive electrode active material was completely leached by using the strongly acidic leaching solution by the method described in the non-patent document 1 by Jinqiu Xu, et al, the Li/Co molar ratio of the dissolved solution before dialysis was about 1. When leaching is performed under a strongly acidic condition at pH≦1, the Li/Co molar ratio is approximately 1.0. This is because the entire composition of lithium cobaltate is dissolved and, if dissolution is terminated in the course of the process, the Li/Co molar ratio scarcely changes. On the other hand, in this embodiment, the Li/Co molar ratio is improved to 4 or more when a weakly acidic leaching solution at pH 4≦1≦7 is used. This is because Li is dissolved preferentially to Co (although detailed reaction mechanism is not apparent). The weakly acidic leaching solution may also be a buffer solution with addition of a material having a buffering effect to pure water. The Li/Co molar ratio was 4 in a case of a phthalic acid buffer solution (mixture of phthalic acid and potassium phthalate) adjusted to pH at 4. The Li/Co selectivity is further improved when a redox potential controller is added to the weakly acidic solution. When a treating solution prepared by adjusting an aqueous hydrogen peroxide to a weak acidity of pH at 4 was used as the leaching liquid, the Li/Co molar ratio was improved greatly as 335. Further, the Li/Co molar ratio was as high as 121 also in a case of adding carbon dioxide to ozonized water.

For the lithium selective leaching solution in this embodiment, an aqueous solution prepared by dissolving, for example, ozone, hydrogen peroxide, or peracetic acid, can be used. Such a solute acts as an oxidizer. Generally, in the recovery of the valuable metals from batteries, a mineral acid at high concentration is used for leaching lithium cobaltate to perform complete dissolution. In this embodiment, the mineral acid at a high concentration is not used and, further, the upper limit of the leaching temperature is defined as 30° C. If the temperature is much higher than 30° C., the rate of spontaneous decomposition of ozone or hydrogen peroxide increases to form a solute not contributing to the dissolution of the positive electrode active material and the solute is consumed wastefully.

When a leaching solution at a pH of 4 to 7 and a redox potential of 0.3 to 0.4 V is used, a high Li/Co molar ratio tends to be obtained. When the value of pH becomes less than 4, the dissolution rate of the positive electrode active material increases to increase the Li dissolution rate and the recovery rate tends to be higher. However, the Co dissolution rate also increases, tending to lower the Li/Co molar ratio. In a range of the concentration of hydrogen peroxide in this embodiment illustrated in FIG. 3, leaching of cobalt starts to increase where the concentration of the aqueous hydrogen peroxide is higher than 15%. The Li/Co molar ratio is high in a region where the concentration of the aqueous hydrogen peroxide is lower than 20%. In this embodiment, since a redox potential is 0.3 V at the concentration of aqueous hydrogen oxide of 15% and a redox potential is 0.4 V at the concentration of aqueous hydrogen peroxide of 20%, it has been found that a high Li/Co molar ratio is obtained in a range of the redox potential of 0.3 to 0.4 V.

In the recovered solution (A) obtained by selective leaching, lithium is leached selectively and the solute used in the leaching solution (specifically, hydrogen peroxide or ozone) spontaneously disappears. Spontaneous disappearing means that a concentration of a certain effective ingredient lowers to about one-half or lower of an initial concentration without adding any chemical material for promoting neutralization or decomposition. For example, hydrogen peroxide is spontaneously decomposed into water molecules and oxygen molecules and ozone is spontaneously decomposed into oxygen molecules. Most of generated oxygen molecules are released out of the solution. Further, carbon dioxide is vaporized and released out of the solution. Accordingly, since the solution becomes neutral by spontaneous disappearing, a conventional neutralization treatment used after the acid leaching is no more necessary (S105).

Referring to the succeeding operation, since the solution of the valuable metals obtained after the leaching treatment is a solution of a strong acid at a high concentration in the method proposed in the non-patent document 1 Jinqiu Xu, et al, a so-called pH control of mixing a great amount of an alkali is inevitable before the recovery of lithium as lithium carbonate. However, ozonized water or aqueous hydrogen peroxide used in this embodiment is a spontaneously disappearing solution and the pH of the recovered solution (A) after the leaching treatment of lithium cobaltate is weakly alkaline, that is, at a pH of about 9 to 11. When the thus obtained recovered solution (A) is treated, for example, by mixing an alkali metal-free carbonate such as calcium carbonate or gaseous carbon dioxide without neutralization such as pH adjustment, lithium can be precipitated and recovered as an alkali metal-free lithium carbonate (S106).

Since not only the neutralization is unnecessary but also ozone, hydrogen peroxide, gaseous carbon dioxide, or the like spontaneously disappears and the solution left after lithium recovery becomes water, it can be utilized again for aqueous hydrogen peroxide or ozonized water and, as a result, a liquid waste-free metal recovery method can be constructed.

Transition metal ingredients are recovered from a residue (B) obtained in the separation of Li and transition metal at S104 (S107). In this embodiment, since the positive electrode active material mainly comprising lithium cobaltate is treated, the residue (B) obtained by the treatments so far contains a small amount of leached lithium and cobalt. When they are separated at a high purity or when the residue contains transition materials other than cobalt and they are intended to be separated and recovered for every kind of metals, they can be separated and recovered for every kind of transition metal elements by treatment utilizing the difference in the dissolution property of hydroxides of respective metal elements, basically, by repeating pH control→settling and recovery (S108). Co, Ni, Mn, and Fe can be separated as hydroxides, and precipitated and recovered by pH control of the solution also in a case where the positive electrode contains lithium compounds other than LiCoO₂, for example, olivine type positive electrode active material such as LiNiO₂, LiMnO₂, Li(Ni_(1/3)Co_(1/3)Mn_(1/3)) O₂, LiCoPO₄, LiFePO₄, LiCoPO₄F, and LiFePO₄F.

Second Embodiment

The outline for the method of recovering valuable metals in this embodiment is to be described. The method of recovering the valuable metals in this embodiment is basically identical with that of the first embodiment. This embodiment is different from the first embodiment in that a spontaneously disappearing oxidizer and a buffer solution of suppressing the spontaneous disappearing rate of the oxidizer are used as a weakly acidic leaching solution in the leaching treatment at S103 in FIG. 2. This can attain compatibility between a high Li/Co molar ratio and a high Li recovery rate in the lithium leaching reaction.

As the leaching solution in this embodiment, a mixture, for example, of an ozonized water at a dissolved ozone concentration of 150 ppm as a spontaneously disappearing oxidizer, and an acetic acid buffer solution (0.1M, pH 4.7) or a phthalic acid buffer solution (0.1M, pH 4.0) as a buffer solution comprising a carboxylic acid and a salt thereof can be used, but the kind of the oxidizer, the kind of the carboxylic acid, and the concentration thereof are not restricted to them. A positive electrode active material is added to the mixed solution of the ozonized water and the buffer solution, and stirred for about 20 minutes to leach lithium (S103).

In this embodiment, the leaching solution and the residue are separated for terminating the leaching treatment (S104). As the separation method, centrifugal separation, filtration, etc. can be adopted. Separation and recovery are possible in the centrifugal separation by treating, for example, at a room temperature, under 10,000 rpm, for 30 seconds.

The recovery solution (A) formed by the selective leaching described above is obtained as a lithium concentrate (S105) in which the oxidizer (ozone) used in the leaching solution disappears spontaneously. The Li/Co molar ratio in the recovery solution (A) obtained in this embodiment was 4243 in a case of using an acetic acid buffer solution and a high Li/Co molar ratio was obtained in the same manner as in the first embodiment. Further, as illustrated in FIG. 4, while the lithium recovery rate was 18% in a case of using only the ozonized water as the leaching solution, it is improved to 28% in a case of using the ozonized water and the phthalic acid buffer solution and, further, value as high as 98% is obtained in a case of using a mixture of the ozonized water and the acetic acid buffer solution. As described above, a lithium recovery method at a high lithium recovery rate and a high Li/Co molar ratio can be established by using the ozonized water and the buffer solution.

In the recovery of lithium, it is more preferred that both of the lithium recovery rate and the Li/Co molar ratio in the recovered ingredients are higher and it is preferred that the former is about 100%. The reason why such high lithium recovery rate can be obtained by the spontaneously disappearing oxidizer and the buffering solution is to be described specifically later.

Phthalic acid is dissociated in an aqueous solution in accordance with the chemical formula 1 and the chemical formula 2 shown below. The acid dissociation constant kPa (logarithm of the reciprocal of the dissociation constant) in the chemical formula 1 is 2.94 and pKa in the chemical formula 2 is 5.41. In a preferred pH range of: 4<pH<7 described in the first embodiment, equilibrium in the chemical formula 1 shifts to the right substantially completely. In the chemical formula 2, it shifts to the right or the left depending on pH, or ingredients in the both sides are present each in an meaningful amount. When the phthalic acid buffer solution is used, pH is kept in a preferred range by the equilibrium reaction depending on pH and the ozone decomposition is suppressed to improve the lithium recovery rate from 18% to 28%.

C₆H₄(COOH)₂

C₆H₄(COOH)COO⁻+H⁺  (Chemical formula 1)

C₆H₄(COOH)COO⁻

C₆H₄(COO⁻)₂+H⁺  (Chemical formula 2)

In this embodiment, when an acetic acid buffer solution is used as the buffer solution, the lithium recovery rate is outstandingly improved. The reason is to be described below.

In the preferred pH range described above, most of phthalic acid components are dissociated. When the phthalic acid is dissociated into C₆H₄(COOH)COO⁻, an electron density on carboxyl groups is higher. Since the carboxylate ion is an electron donating group, the electron density on an aromatic ring increases as a result. Further, since the phthalic acid has an aromatic ring in the molecular structure as a weak electron attracting group, the electron density tends to increase. When the electron density on the aromatic ring is high, the electrons tend to be deprived by an oxidizer. Accordingly, when dissociated phthalate ions and ozone as the oxidizer are present together, ozone may sometimes oxidize a portion of the phthalate ions. In this case, ozone after oxidizing the phthalic acid is consumed without reaction to LiCoO₂. Further, the oxidized phthalic acid becomes a neutral radical and, since the radical cannot contribute to the reactions in the chemical formula 1 and the chemical formula 2, an effective concentration of ions constituting the buffer solution is lowered to lower the buffering effect of the solution. When the reaction of leaching LiCoO₂ proceeds in a state where the buffering function is lowered, OH⁻ is formed and therefore pH of the reaction solution shifts to the alkaline side. When the reaction solution becomes alkaline, a portion of ozone is decomposed without reaction to LiCoO₂.

As the result described above, the reaction of leaching LiCoO₂ no more proceeds. The buffering solution providing the same effect includes, for example, benzoic acid.

Compared with the phthalic acid, since an acetic acid has no site where the electron density increases locally in the molecular structure as the aromatic ring, electrons are less deprived by the oxidizer, the acetic acid is less oxidized even when ozone as the oxidizer is present together. That is, since the acetic acid is less decomposed, the buffering effect in the reaction solution can be maintained for a long time.

Further, since the acetic acid buffer solution can maintain the buffering effect in the reaction solution and the pH of the reaction solution does not become alkaline even when the dissolving reaction of the positive electrode active materials occurs, ozone can be suppressed from disappearing spontaneously due to increase of pH and, as a result, ozone is utilized effectively to the leaching reaction of LiCoO₂ and a high lithium recovery rate can be attained.

FIG. 5 illustrates a result of actually measuring the change of a dissolved ozone concentration in the leaching reaction of LiCoO₂. As seen in FIG. 5, when the positive electrode active material is dissolved only by ozonized water, the dissolved ozone concentration lowers abruptly in initial several minutes and the dissolved ozone concentration is lowered to about 5 ppm for a leaching time of 10 minutes. On the other hand, when an acetic acid buffer solution is added to the ozonized water, the dissolved oxygen concentration at the leaching time described above is 50 ppm and it can be seen that lowering of the dissolved oxygen concentration is suppressed. That is, when the positive electrode active material is leached, lowering of the dissolved ozone concentration can be suppressed to maintain the concentration by the addition of the acetic acid buffer solution to the ozonized water. Then, by maintaining the dissolved ozone concentration at a high level for a long time, the leaching reaction of lithium from the positive electrode active material can be performed efficiently.

As will be apparent from the consideration described above, the effect of the acetic acid is not restricted only to that obtained by the acetic acid but may be obtained also by any other carboxylic acid having a carboxyl group and containing an aromatic ring.

In this embodiment, while the result for the acetic acid buffer solution is shown, other aliphatic monocarboxylic acids such as propionic acid, butanoic acid, and pentanoic acid can be used according to the study made by the present inventors. Acid having the number of carbon atoms constituting the carbon chain larger than that of the acids described above are not suitable to the practical use since the solubility to water is lowered. The concentration of the buffer solution used herein is generally about 0.1 mol/L and the concentration is preferably 0.001 mol/L or higher at the lowest. The solubility of materials having a small number of carbon atoms (values at 20° C. shown by the mass of a solute dissolved in 100 g of water and a molar concentration in this state) is 37 g/100 g (0.005 mol/L) for propionic acid, 5.6 g/100 g water (0.032 mol/L) for butanoic acid, and 2.4 g/100 g water (0.075 mol/L) for pentanoic acid. The oxidative decomposition rate of aliphatic monocarboxylic acids is in the order of: butanoic acid >propionic acid >acetic acid. The oxidative decomposition rate is lower as the number of carbon atoms is smaller, and the performance capable of attaining compatibility between a high Li/transition metal separability and a high lithium recovery rate is more excellent as the number of carbon atoms is smaller. However, those acids having a number of carbon atoms of 0 in other portion than the carboxyl group are not preferred. For example, formic acid has an aldehyde group. Since the aldehyde group is oxidized by the oxidizer and the oxidizer is consumed in the reaction of oxidizing the buffer solution, formic acid is not suitable. Oxalic acid is not suitable since pKa is as low as 1.25 and oxalic acid is classified as a strong acid. For polyhydric aliphatic carboxylic acids, oxalic acid, succinic acid, tartaric acid, citric acid, malic acid, and malonic acid can be used, in view of pKa and solubility. Further, a customary glycine may also be used as the buffer solution.

As described above, buffer solutions comprising materials having the number of carbon atoms of 1 to 4 (not including carbon atoms in the carboxyl group) and the acid dissociation constant of: 4<pH<7 are preferred with a view point of the solubility.

When the buffer solutions described above are used, compatibility can be attained between high Li/transition metal separability and high Li recovery rate.

For example, Chinese Patent Laid-Open No. CN101673859A (Patent document 2) discloses a method of recovering lithium and cobalt by using an organic acid such as citric acid, succinic acid, malic acid, etc. for a solution of leaching a positive electrode active material. However, since the buffer solution is not used, pH is low and for example, pH is 1 or less in a solution of citric acid at a concentration of 1.25 mol/L described in Example 1 of the Patent document 2. Accordingly, LiCoO₂ is entirely dissolved and recovered lithium and cobalt are at a low purity, and a separation operation is necessary for recovering only lithium. On the other hand, as a result of studies made by the inventors, in the embodiment, a weakly acidic leaching solution is suitable and obtained metals can be purified to a high level by selectively leaching only lithium from the positive electrode active material and leaving only cobalt in the residue after leaching by leaching at a weakly acidic state.

Further, as the buffer solution, not only the carboxylic acid but also other buffer solutions may be used providing that the buffer solution exhibits an acidity (4<pH<7). As other buffer solutions than the carboxylic acid, a buffer solution, for example, of phosphoric acid and a salt thereof can be used. They include, for example, a buffer solution comprising sodium dihydrogen phosphate and disodium hydrogen phosphate but they are not restrictive but other compositions may also be used. When phosphor is recovered after recovering Li from the acidic solution in which Li is dissolved selectively, phosphor can be utilized again as a solute of the buffer solution. The phosphor separation and recovery method includes separating operations such as methods of using dialysis membrane separation, acid retardation, and ion exchange resin.

Further, while ozone was used as the spontaneously disappearing oxidizer in this embodiment, aqueous hydrogen peroxide may also be used instead.

While LiCoO₂ was used as the positive electrode material in this embodiment, when the positive electrode active material contains lithium compounds other than LiCoO₂, for example, olivine type positive electrode active materials such as, LiNiO₂, LiMnO₂, Li (NiCoMn)O₂, LiCoPO₄, LiFePO₄, LiCoPO₄F, and LiFePO₄F, etc., Co, Ni, Mn, and Fe can be separated as hydroxides and recovered by precipitation by pH control of the liquid. The operation of recovering transition metals in this case is performed by the same treatments as those in S107 and S108 of the first embodiment, and the method is identical with that of the first embodiment.

As described above, a metal recovery method capable of attaining compatibility between the high Li/transition metal separability and the high lithium recovery rate can be provided by the combined use of the oxidizer and the buffer solution having the effect of suppressing the spontaneous disappearing rate of the oxidizer and selectively leaching lithium. 

What is claimed is:
 1. A metal recovery method of recovering metals from a positive electrode active material of a lithium ion battery containing lithium and transition metal elements, comprising: leaching valuable metals contained in the positive electrode active material into an acidic solution; and recovering lithium from the acidic solution where the valuable metals are leached, wherein the acidic solution is at pH of 4 to
 7. 2. A metal recovery method according to claim 1, wherein the acidic solution contains a redox potential controller.
 3. A metal recovery method according to claim 2, wherein the acidic solution further contains a pH controller.
 4. A metal recovery method according to claim 2, wherein the redox potential controller comprises hydrogen peroxide.
 5. A metal recovery method according to claim 2, wherein the redox potential controller comprises ozone.
 6. A metal recovery method according to claim 3, wherein the pH controller comprises carbon dioxide.
 7. A metal recovery method according to claim 1, wherein one of the solute of the acidic solution, the redox potential controller or the pH controller is a material that spontaneously disappears from the solution.
 8. A metal leaching solution of leaching metals from a positive electrode active material of a lithium ion battery containing lithium and transition metal elements, wherein pH of the solution is from 4 to 7 and the solute of the solution is a spontaneously disappearing solute.
 9. A metal recovery method of recovering metals from a positive electrode active material of a lithium ion battery containing lithium and transition metal elements, comprising: leaching lithium contained in the positive electrode active material into an acidic solution containing a spontaneously disappearing oxidizer and a buffer solution; and recovering lithium from the acidic solution in which lithium is leached.
 10. A metal recovering method according to claim 9, wherein the buffer solution contains a carboxylic acid and a salt thereof as a solute.
 11. A metal recovering method according to claim 10, wherein the carboxylic acid is an aliphatic carboxylic acid having a carboxyl group and not containing an aromatic ring.
 12. A metal recovering method according to claim 11, wherein the carboxylic acid has a number of carbon atoms of 1 to 4 (excluding carbon atoms in the carboxyl group).
 13. A metal recovering method according to claim 10, wherein the carboxylic acid is an organic acid or glycine.
 14. A metal recovering method according to claim 9, wherein the buffer solution contains phosphoric acid and a salt thereof as a solute.
 15. A metal recovering method according to claim 9, wherein the spontaneously disappearing oxidizer comprises ozone or aqueous hydrogen peroxide.
 16. A metal recovering method according to claim 9, wherein the acidic solution in the leaching step is at a pH of 4 to
 7. 17. A metal leaching solution of leaching metals from a positive electrode active material of a lithium ion battery containing lithium and transition metal elements, wherein the leaching solution contains a spontaneously disappearing oxidizer and a buffer solution and a pH of the solution is 4 to
 7. 